1. Field of the Invention
The present invention relates generally to telephone circuits for ring trip detection and, more particularly, to monolithically integrated telephone circuits for ring trip detection that can constitute an interface between the user's telephone line and the user's circuit line under the control of the telephone exchange control equipment.
2. Discussion of the Related Art
A user's telephone apparatus is coupled to the telephone exchange by means of a line whose terminals are coupled in parallel both to the conversation circuit and the ringing mechanism.
A switch is coupled in series with the conversation circuit which is turned off when an off-hook condition, i.e. when the receiver or headpiece is removed from the body of the telephone, takes place. A capacitor is coupled in series with the ringing mechanism to decouple the ringing mechanism from the direct current line. The direct current line is in fact powered by a direct current generator coupled in series with an alternating current generator which represents the ringing signal generator when the telephone exchange control equipment sends a call signal to the user. Therefore, when there is call, a d.c. component superimposed on a ringing alternating current appears on the line in the off-hook condition.
The ringing signal generally has a frequency of 25 or 50 Hz, with an effective voltage value rather high (60 V to 80 V) relative to the normal telephone conversation signals. If the ring signal is not immediately interrupted at the moment of the off-hook, it is converted by the conversation circuit to a high intensity acoustical signal, which can be detrimental to the hearing of the user.
When electronic circuits have been introduced for the detection of the off-hook condition directly in the user's circuits in the exchange, the problem has risen in recognizing immediately the off-hook condition from the variations of the line current by sensing a possible presence of a d.c. component on the user's telephone line in order to be able to interrupt in a timely manner the generation of the ringing signal. With this recognition, the central control equipment can interrupt the sending of call signals.
In effect, it is not easy to execute such a sensing when the a.c. current due to the ringing signal has an effective value much larger than the value of the direct current present after the off-hook condition is established. The large diversity of impedances that can exist in the various lines and the user's telephone apparatus also makes it necessary to design circuits for the ring trip detection that have sufficient sensitivity, taking into account the worst case that can be established. If the line current is only in the alternating state when the off-hook has not yet occurred in the ringing phase, the line current wave form is comprised of a succession of positive and negative parts which are symmetric in respect to the zero amplitude reference. This symmetry occurs even when the wave form is not perfectly sinusoidal as, for example, when there are non-linear components in the electric circuits coupled to the line.
Therefore, the duration time of the half-waves of current, positive and negative, are equal. The areas defined by the wave form in the positive and negative half-waves are equal.
When the off-hook condition takes place and the conversation circuit direct current is added to the alternating current of the ringing signal in the half-wave periods in which the sign of the alternate current is equal to that of the direct current, the peaks of the wave form of the line current in which the alternating current and the direct current are found to be larger than those peaks in which the alternating current and the direct current have opposite signs.
Therefore, when the off-hook condition occurs, the positive half-wave periods of the line current in the ringing phase have a different duration than that of the negative half-wave periods. The area defined by the wave form of the current in the positive half-wave periods also is found to be different from that of the negative half-wave periods. The difference in these parameters are determined by the amplitude of the direct current. Two distinct procedures are utilized in the related art for the ring trip detection. They are based on the detection of the differences between the positive half-wave and the negative half-wave of the line alternating currents as altered by any direct current component.
A first procedure based on a comparison of the duration times of the positive and negative half-wave periods for one or more periods is, for example, made by counting the number of impulses having a predetermined duration time that can occur in each half-wave periods with one of the methods known to a person skilled in the art. The control devices that actuate such procedure do not however guarantee a sure detection of the off-hook condition in all of the conditions of the telephone network that can arise in practice. In fact, in a telephone line, some stray current is always present that, added to the line alternating current, can shift the point of separation between the positive and negative half-wave periods of the current wave form or make the position of this point uncertain. In this way, an "indetermination interval" of the detection is created whose amount depends on stray current which, in turn, depends on the different lengths and impedances of the lines. Therefore, such control devices can be effectively utilized only when the conditions of the line guarantee an indetermination interval under predetermined values.
The second procedure is actuated by measuring, during one or more periods, the total area defined by the wave form, attributing a positive value to the positive peak area and a negative value to that of the negative peak area. If the value of the total area is not equal to zero and exceeds a predetermined threshold value for certain detection, this value signifies that there is a direct current component in the line alternating current, i.e. the off-hook has taken place, and a command can be given to the exchange equipment, to interrupt the sending of call signals. Such a measure, defined as a measure of an "average value", can be effected by means of an integrator circuit that generates a signal proportional to the integral of the line current for one or more full periods. The generated signal has a zero value, or a value under a predetermined threshold only when, in the ringing phase, the off-hook condition has not yet taken place.
Such an integrator, as is well known to a person skilled in the art, can be realized simply with a RC type network. When, however, monolithically integrated telephone circuits are used to actuate this second procedure, there is a problem of sensitivity of the recognition device. In fact, for the integration during one or more periods, the signal value generated by the integrator during the period can be much higher than the final value that indicates an eventual off-hook condition and, especially when the line current is not perfectly sinusoidal, and has high peaks, the signal value can not be compatible with the limited dynamics of the integrated circuits that are used. On the other hand, because the off-hook direct current has a much lower value than the ringing alternate current, the line current cannot be altered without lowering the sensitivity of detection.